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Colorful Arcturus

One of the most colorful points of light in the night sky stands high in the south as the color of twilight begins to fade this evening.

Arcturus is the leading light of Bootes, the herdsman. It’s also the brightest star in the sky during the evening hours, so it’s hard to miss.

It’s not just the star’s brightness that helps it stand out, though — its color is pretty eye-catching, too. The star shines yellow-orange — the result of its surface temperature. The hottest stars shine blue-white. Mid-range stars like the Sun are yellow-white. And cool stars are orange or red. Orangey Arcturus is a couple of thousand degrees cooler than the Sun.

Just because a star is cool on the outside doesn’t mean that it’s also cool on the inside, though. The core of Arcturus, which generates the star’s energy, is much hotter than the core of the Sun. That’s because the star is nearing the end of its life. It converted its original hydrogen fuel to helium. That caused the core to shrink, making it hotter — hot enough, in fact, to begin burning the helium to make even heavier elements. The extra heat in the core pushes outward on the surrounding layers of gas, causing them to expand and cool — giving Arcturus a yellow-orange surface.

Over many millions of years, the surface will become redder as Arcturus continues to evolve. Finally, the star will cast its outer layers into space. Only its hot but tiny core will remain — a white dwarf.

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